Reactors have a fairly slow rate of cooldown from operational temperatures. In order to maintain a reactor safely, the reactor must be cooled to a temperature that will allow maintenance workers to open and interact with the reactor. Given the costs associated with downtime with these vessels and reactors, a need exists to cooldown reactors in an accelerated manner.
Vessel reactor systems have benefited from accelerated cooldown services. Typically this process is done in one of two ways. First, cool nitrogen gas can be passed through a reactor system. As the gas moves though the reactor, it exchanges heat with any matter it comes into contact with, causing a faster than normal, or accelerated cooldown. In the alternative, cryogenic nitrogen fluid has been pumped into the gas stream within a specially designed reactor system. The nitrogen is vaporized by the warm gas stream and forms mixed gas at a lower temperature. This cool gas mixture is used in the same manner as the gaseous cooldown to accelerate the cooling of the reactor system.
In order to create the cool gas required for a gaseous cooldown, the cryogenic liquid nitrogen is vaporized and heated to a temperature that can be tolerated by the metallurgy of the reactor in question. The efficiency of a liquid cooldown is higher, because the energy to vaporize and heat up the gas from an extremely cold temperature are extracted from the reactor and not injected by the nitrogen equipment. As a general rule a cooldown with liquid is about 3.5 times more efficient than a gas cooldown. As a result it costs less than about 30% to cooldown a reactor with liquid as compared to gas.
There are several limitations with the liquid cooldown that restrict its application with in industry. The metallurgy of the system must be compatible with cryogenic temperatures. Pipes made from stainless steel with high nickel content can tolerate liquid nitrogen temperature. Moreover, the system must have a carrier gas in order to vaporize and carry the gas mixture throughout the reactor system. Furthermore, a system that recycles its gas can more fully utilize the cooling power of the liquid. Finally, cryogenic liquid will destroy most reactor systems if not properly sparged and mixed.
There are also limitations on gas cooldown methods. The limiting factor in gas cooldown methods is the amount of product required to cool down any substantial reactor. It is the transport of the liquid to site which is more of a factor than the bulk cost of the nitrogen. This creates an effective radius of application. Beyond this radius, while accelerating the cooling of a reactor is attractive, the costs of doing the operation out weigh the benefits in all but the most extreme situations. Therefore, a need exists to accelerate the cooldown of reactors and vessels using a liquid medium that does not require the application of expensive cryogenic piping in a method that will not damage the carbon steel of these systems.
The prior art has only used carbon dioxide that was actually injected right into the reactor to control the temperature of an exothermic reaction. Direct injection into a reactor or similar vessel does not produce good flow characteristics during shutdown. Without even distribution of a cooldown medium, the cooldown of the reactor will take longer. There exists a need to be able to take advantage of the open space, preferably with a high velocity gas, by putting it into the feed pipe of the reactor. Moreover, a need still exists for a system and a method of its use that will allow for using existing piping to provide for well distributed cooling method using the existing pipeline to accelerate the cooldown of a reactor during downtime and maintenance rather than attempting to control the reaction itself. The prior art has failed to offer an efficient and safe manner of accelerating the cooldown of a reactor so that the reactor will be safe to enter as quickly as possible.